1. An Technical Field Affiliated with the Invention
The present invention concerns the manufacturing methods of an anisotropic magnet powder, the precursory anisotropic magnet powder and it manufacturing method, as well as a bonded magnet made from this powder.
2. The Conventional Technique
Magnets are widely used in many of the machines in our surroundings, including various types of motors. There is a need for a stronger permanent magnet in order to reduce the weight, thickness and length of an the increase efficiency of these machines. A rare earth element magnet (RFeB magnet) mainly composed of Nd2Fe14B has been attracting much attention as a candidate for such a permanent magnet, and its range of applications has been expanded greatly. For example, it is being considered as a motor magnet in various types of machines in the automobile engine room. Here it is desired that the magnet have a high heat resistance because the temperature inside the engine room exceeds 100xc2x0 C.
However, the precursory anisotropic magnet powder (RFeB magnetic powder) has large temperature dependence (temperature coefficient), which causes a poor heat-resistance. The coercivity decreases rapidly at the high range of temperatures. It has been difficult to readily improve the temperature dependency so far. A remedy for this may be the use of an anisotropic magnet powder which originally has a very large coercive force (iHc), so that the magnet may keep a large enough coercive force even at the high range of temperatures. Such an anisotropic magnet powder and its manufacturing methods have been disclosed in Japanese laid-open patent numbers 9-165601 and 2000-96102.
Concretely, in Japanese laid-open patent number 9-165601, a manufacturing method of an anisotropic magnet powder by HDDR (hydrogenationxe2x80x94decompositionxe2x80x94desorptionxe2x80x94recombination) method has been shown using an ingot to which a minute amount of Dy was added to the molten RfeB alloy, resulting in an average crystal radius ranging from 0.05-1 xcexcm.
However, when the inventors actually tried to manufacture this anisotropic magnet powder, a stable coercivity could not be achieved due to the limited amount of Dy additive and the method was also difficult to mass-produce. In addition, the coercivity of the anisotropic magnet powder produced by this method was at most 16 kOe (1272 kA/m).
In general, a desirable anisotropic magnet powder should have large values for both coercivity (iHC) and degree of anisotropy (Br/Bs), where (Br) is the residual magnetic flux density and (Bs) is the saturation magnetic flux density. However, while the addition of Dy is efficient for improving the coercivity, it will also reduce the rate of HDDR reaction causing a decline in the degree of anisotropy. For these reasons, until now, these values have not been optimized at the same time.
In Japanese laid-open patent number 2000-96102, another manufacturing method of an anisotropic magnet powder is described in which and a Dy alloy powder is mixed with an already produced anisotropic magnet powder, and this mixture is heat treated under a vacuum or inactive gas atmosphere so that the anisotropic magnet powder receives a thin coating of Dy on its surface. In this way, an appropriate amount of Dy can be coated on the powder surface, increasing the coercivity to as high as 18 kOe (1432 kA/m) and maintaining a high degree of anisotropy.
However, because the starting material in this method is an anisotropic magnet powder such as Nd2Fe14B, the control of oxidization is difficult while Dy coating, there is substantial variation in the end powder""s performance and quality. Thus a magnet made from this anisotropic magnet powder an uncontrollable loss of magnetization due to structure change, as will be discussed later, and a permanent magnet with stable heat-resistance could not be obtained.
1. A Problem to Solve in the Invention
The invention is proposed in light of the circumstances stated above, and intends to provide a manufacturing method of an anisotropic magnet powder by which a magnet with an improved coercivity and loss of magnetization due to structure change can be obtained with a high productivity and a constant quality.
The invention is also intended to provide a suitable precursory anisotropic magnet powder and to provide its manufacturing method, as well as to provide a bonded magnet with a high degree of permanent demagnetization.
2. A Means to Resolve the Problem
(1) The inventors devoted themselves to the resolution of the problem, making a systematic study on it with repeated trial and error, and finally found out that oxidation is inhibited if diffusion heat-treatment is carried out after blending a RFeB hydride powder material with R1 element diffusion powder containing Dy, while the process results in an anisotropic magnet powder in which Dy is uniformly diffused on the surface of and inside the powder. That is how the inventors came to develop the present invention of a manufacturing method of anisotropic magnet powder.
The manufacturing method of the present invention comprises the following processes;
A blending process of RFeB hydride (RFeBHx) powder, which is mainly composed of rare earth elements including yttrium (Y) (hereafter referred to as xe2x80x9cRxe2x80x9d), boron (B) and iron (Fe), with diffusion powder, which is composed of a simple substance, an alloy, a compound or a hydride of one or more elements in an elemental group which includes dysprosium (Dy), terbium (Tb), neodymium (Nd) and praseodymium (Pr) [hereafter referred to as xe2x80x9cR1 elementsxe2x80x9d];
a diffusion heat-treatment process in which R1 elements are diffused uniformly on the surface and the inside of the RFeBHx powder; and
a dehydrogenation process (the second evacuation process) in which hydrogen is removed from the mixture of the powder after the diffusion heat-treatment process.
When RFeBHx powder and diffusion powder are mixed together in a blending process, R and Fe are difficult to oxidize compared to a conventional RFeB powder because the RFeBHx powder contains hydrogen. For this reason, in the following diffusion heat-treatment process, the diffusion of Dy, Tb, Nd and Pr (R1 elements) will diffuse into the surface and the inside of the RFeBHx powder with oxidization being sufficiently inhibited.
Furthermore, the speed of diffusion of R1 elements into the surface and the inside of the RFeBHx powder is enhanced by diffusion into the crystal particle boundaries and into the crystal particles, leading to uniform addition of R1 elements.
An anisotropic magnet powder with a large coercivity and a consistent quality can be achieved with RFeBHx powder material that can hardly be oxidized, and diffusion of R1 elements with inhibited oxidization. A bonded magnet molded from the anisotropic magnet powder obtained by this method will have an improved loss of magnetization due to structure change. This loss of magnetization is calculated using the magnetic flux when the sample magnet is initially put in a magnetic field and the magnetic flux after the sample is left under air atmosphere for 1000 hours at 120xc2x0 C., where the magnet does not recover when remagnetized. And the loss of magnetization is a comparison to the initial magnetic flux.
Furthermore, the inventors of the present invention developed a suitable RFeBHx powder, or precursory anisotropic magnet powder, for manufacturing of such an anisotropic magnet powder. The precursory anisotropic magnet powder is the RFeB hydride (RFeBHx) powder which is mainly composed of rare earth elements including yttrium (Y), boron (B) and iron (Fe) and is characterized by an average crystal radius ranging from 0.1-1.0 xcexcm.
The use of the RFeBHx powder, or precursory anisotropic magnet powder, makes it easier to manufacture, for example, the anisotropic magnet powder stated above.
The reasons that the range of 0.1-1.0 xcexcm was chosen as the average crystal radius is the difficulty to manufacture RFeBHx powder whose average crystal radius is less than 0.1 xcexcm, and the poor coercivity of anisotropic magnet powder made from RFeBHx powder whose average crystal radius is greater than 1.0 xcexcm.
The average crystal radius was determined via TEM (transmission electron microscope). Crystal particles of RFeBHx powder were observed, two-dimensional image processing was carried out, equivalent cross sections of the area circles and crystal particles were assumed and the average radius was calculated.
For the precursory anisotropic magnet powder and the anisotropic magnet powder described above, there are no particular restrictions to the particle shape or size, so both fine and coarse powders are available. When the RFeB material is in a powder state, it is not necessary to establish an additional crushing process, however if a crushing process is carried out, anisotropic magnet powder or precursory anitsotropic magnet powder with a narrow distribution of particle radius can be obtained.
In addition, by using the anisotropic magnet powder mentioned above, a bonded magnet with an improved loss of magnetization due to structure change was invented. A bonded magnet is mainly composed of rare earth elements including yttrium (Y), boron (B) and iron (Fe), made of an anisotropic magnet powder whose average crystal radius is 0.1-1.0 xcexcm, was developed with a degree of anisotropy (Br/Bs) (the ratio of the residual magnetic flux density (Br) to the saturation magnetic flux density (Bs)) greater than 0.75, and a loss of magnetization less than 15% due to structural changes.
Because the bonded magnet is made of an anisotropic magnet powder whose crystal particle is small with a high degree of anisotropy, the bonded magnet not only has greater magnetic characteristics, but also has improved heat-resistance for its low loss of magnetization due to structural changes, which is less than 15%.
A bonded magnet with a loss of magnetization due to structure changes greater than 15% will have poor heat-resistance that is unsuitable for long-term use under high-temperature conditions. The degree of anisotropy, which is given by the ratio of Br to Bs, depends on the composition (volume %) of an anisotropic magnet powder. For example, when the anisotropic magnet powder consists of only Nd2Fe14B, an appropriate Bs is 1.6 T, while with the addition of Dy, Bs is reduced to 1.4 T due to ferromagnetism.
The present invention consists not only of an RFeBHx powder, but also consists of the manufacturing method of the precursory anisotropic magnet powder.
The manufacturing method of the present invention comprises the following processes;
A low-temperature hydrogenation process in which a RFeB powder, which is mainly composed of rare earth elements including yttrium (Y), boron (B) and iron (Fe), is maintained under hydrogen gas atmosphere at a temperature lower than 600xc2x0 C.;
a high-temperature hydrogenation process in which the powder is maintained under hydrogen gas atmosphere with pressure ranging from 0.1-0.6 MPa and temperature ranging from 750-850xc2x0 C.; and
the first evacuation process in which the powder is maintained under hydrogen gas atmosphere with pressure ranging from 0.1-0.6 kPa and temperature ranging from 750-850xc2x0 C.
Following each process (low-temperature hydrogenation, high-temperature hydrogenation and the first evacuation process) controlled under the proper conditions, a structure transformation in the RFeB material will occur, bringing about homogenized minute crystal particles and RFeBHx powder with a high degree of anisotropy.